The use of self-propelled rotary cutters in machining (cutting) of metal is known in the context of turning operations in which a workpiece is rotated in a lathe. Such use is also known in the context of milling operations.
U.S. Pat. Nos. 2,233,724, 2,513,811 and 4,181,049 pertain to early and more recent examples of the use of rotary cutters in turning operations. U.S. Pat. No. 3,329,065 is an example of the use of such cutters in milling operations.
In the context of machining of metals, self-propelled rotary cutters are annular elements, often called "inserts" because of their replaceability in a suitable carrier or tool body, which are much harder than the metals which they are used to cut. The cutter inserts can be provided in various geometries, but flat washer-like geometries are common and are often favored because their simple geometry is related to reduced cost. A cutter insert is carried in a tool body for rotation about the cutter axis. Either by suitable design of a milling tool body and how it rotatably carries the cutter, or by appropriate positioning of the tool body with its cutter relative to a rotating workpiece in a turning lathe, a circumferential cutter face and an adjacent edge on the cutter element are placed in a desired position relative to a workpiece to be turned or milled. That desired positional relation between the cutter and the workpiece during cutting operations, in which one or the other of the tool and the workpiece are rotated relative to each other and the cutter is moved into the workpiece, causes the friction between the cutter and the workpiece to rotate the cutter about its own axis. That friction is the friction between the cutter and chips of metal formed by forceful contact between the cutter and the workpiece. As the cutter turns about its axis, new portions of the cutter face and edge move into cutting contact with the workpiece, while those portions of the face and edge which have moved out of that contact are able to cool before recontacting the workpiece. That ability of a rotating cutter to stay much cooler than a nonrotary cutter under comparable machining conditions gives a rotary cutter a significantly longer useful life than a nonrotary cutter.
Notwithstanding suggestions in some literature about the utility of self-propelled rotary cutters in boring operations, so far as is known such cutters have actually been used to date only in turning and milling operations. As will be noted more fully in the following detailed description of an exemplary boring tool having self-propelled rotary cutters, turning and milling operations have much in common with each other in terms of how they apply forces to rotary cutter inserts, while boring operations present a meaningfully different regime and set of cutter insert loading conditions and effects. Those differences, and an apparent lack of understanding of them, have presented a barrier to the successful and effective use of self-propelled rotary cutters in metal boring operations.
Metal boring is different from drilling. Boring presumes the presence in a workpiece of a roughly circularly cylindrical hole, cavity or passage the walls of which are to be machined to provide a more cylindrical hole, e.g., having a specified diameter and a desired surface finish. Boring is the operation used to achieve those objectives. Drilling, on the other hand, generally presumes that the workpiece has no hole, cavity or passage in the place of interest; drilling can be the operation used to create such a hole, cavity or passage of specified diameter and, if appropriate, desired surface finish. Bores usually have ratios of diameter-to-length which are much greater than the holes, e.g., created by drilling.
Metal boring operations increasingly present machining conditions in which self-propelled rotary cutters could be used to great advantage but have not been so used to date. For example, in the automobile industry, many factors, notably desires for increased fuel efficiency through the use of lighter vehicles, are stimulating automobile manufacturers to produce engines made predominately of aluminum or other light-weight alloys or other materials (such as compact graphite iron, a special form of cast iron) which enables the engine block to be lighter overall. Aluminum is attractive because it is light and comparatively inexpensive. Aluminum, however, is not known for its wear resistance, especially at high temperatures, unless it is specially, and expensively, alloyed with other materials.
Automotive engine blocks are formed initially by casting processes and then by finish machining processes including drilling, tapping, milling, boring and other machining processes. If cast aluminum is used to form the raw engine block, the walls of the cylinder bores in those engines often are defined by a sleeve of metal which has high wear resistance at high temperatures. The cylinder walls can be formed by machined sleeves inserted into a machined engine block or, more preferably, by generally tubular pieces of such wear resistant material around which the aluminum engine block is formed in the casting of the block. In the latter situation, a desirable material to use in forming the cylinder insert is compacted graphite iron, or "CGI." CGI is a very difficult material to machine due to its high tensile and shear strength which must be overcome by a cutter during machining. To date, so far as known, CGI has not been used as cylinder insert material in production automotive engine blocks. Gray iron (cast iron) is the most common material from which production engine blocks are cast and in which cylinder bores are machined.
Transfer lines are widely used in the production of a finish machined automotive engine block from a raw engine block casting. A transfer line includes a series of machining stations, through which an engine block passes in sequence, and at which one or a few particular machining operations are performed on a block casting. Modern transfer lines are highly automated. Raw block castings are loaded, manually or by robots or other mechanisms into one end of a transfer line and finish machined blocks are unloaded in a similar manner from the other end of the line. Between the ends of a transfer line, the blocks normally are not touched by human hands. Transfer lines function most efficiently when the need to replace worn or dull metal cutting elements at the several machining stations is minimal. Heretofore, when engine blocks have been handled in automated machining transfer lines, the cylinder boring operations frequently, if not commonly, are the operations which limit overall transfer line efficiency. The reason is the need to comparatively frequently replace boring tools in which the metal cutting elements are fixed in the tools and have a short useful life because of the high temperatures they experience as they are used continuously. It is quite common for modern automated transfer lines to have two successive cylinder boring stations, for rough and semi-finish boring respectively, followed by a cylinder honing station. Thus, cylinder boring of engine blocks long has been a troublesome matter in the automobile industry.
In view of the foregoing, it is seen that a significant need has long existed, in the manufacture of light-weight automotive engines in automated transfer lines, among other contexts, for the benefits of self-propelled rotary cutters in metal boring operations. The aspects of this invention which make it useful to the automotive manufacturing industry also make the invention useful in other industries which practice metal boring processes.